I downloaded the plugin from https://github.com/alixandru/q2a-open-login.
On logging through Facebook, it asks me "You previously logged in to onesharedearth with Facebook. Would you like to continue?". On answering yes, it brings me to back to my website, but does not log me in to q2a. The link displays: https://onesharedearth.com/?provider=Facebook&code=5#_=_
If I'm logged out of Facebook before trying this, then after this error I find that I'm logged into Facebook, so it looks like it has been able to authenticate using Facebook, just not into my website.
Suspecting that it may be a permissions issue, I removed 'user_about_me, user_location, user_website' from the permissions requested from Facebook, so I'm only requesting 'email' now. Still the same issue.
I've also applied the workaround suggested in the last answer in http://www.question2answer.org/qa/57432/q2a-open-login-master-no-longer-working-for-facebook?show=57432#q57432. Still the same issue.
Note that I have already registered using the same email into the website - would that cause a conflict in the login using Facebook?
I'm still using the HybridAuth version that comes with the plugin. I tried upgrading to 2.10.0 and also 2.9.2 but they result in other errors.